1. Field of the Invention
The present invention relates to a magneto-optical head utilizing the magneto-optical effect to read information from a magneto-optical disc and a polarizing optical element used in the magneto-optical head. The present invention also relates to a polarizing analyzing device for detecting diffracted light from the polarizing optical element.
2. Description of the Prior Art
As a writable and erasable information carrier, a magneto-optical medium is well known in the market. In recording or erasing information on or from the medium, respectively, the magneto-optical medium exhibits such a characteristics that, when a laser beam is radiated to the disc accompanied by an increase in temperature thereof, the coercive force of the disc is lowered. On the other hand, when reading the recorded information from the mangeto-optical medium, Kerr effect or Faraday effect is utilized. In other words, by utilization of a phenomenon in which, when the laser beam is reflected from or transmitted through the magnetized magneto-optical medium, polarizing plane rotates, information recorded on the magneto-optical medium is read out therefrom by the detection of a slight change in polarizing state of the laser beam. One example of the magneto-optical head assembly used for the information reading by the utilization of Kerr effect or Faraday effect is illustrated in FIG. 1 of the accompanying drawings.
Referring to FIG. 1, reference numeral 1 represents a semiconductor laser source for emitting a laser beam L which is projected onto a magneto-optical medium 6 after having been collimated by a collimator lens 2 and after having subsequently been passed through a beam shaping prism 3, then through a first beam splitter 4 and finally through an objective lens 5. Reference numeral 21 represents a second beam splitter; reference numeral 22 represents a polarizing beam splitter; reference numerals 26 and 27 represent respective first and second photodetectors each comprised of a photodiode; reference numeral 28 represents a four-division photo-detector assembly comprised of four photodiodes; reference numeral 31 represents an error detector for detecting a focusing error and a tracking error; and reference numeral 32 represents a differential detector.
The illustrated magneto-optical head assembly of the construction shown in FIG. 1 operates in the following manner. The laser beam L emitted from the semiconductor laser source 1 is, after having been collimated by the collimator lens 2, passed through the beam shaping prism 3. The beam shaping prism 3 is operable to render the cross-section of the collimated laser beam L to represent a circular cross-sectional shape. The laser beam having passed through the beam shaping prism 3 is, after having passed through the first beam splitter 4, condensed by the objective lens 5 and then projected onto magneto-optical medium 6. The laser beam L so projected onto the magneto-optical medium 6 is reflected back towards the first beam splitter 4 through the objective lens 5.
When information is being written on the magneto-optical medium 6, the laser beam has its polarizing plane rotated under the influence of Kerr effect and subsequently enters the first beam splitter 4 after having passed through the objective lens 5. The reflected laser beam entering the first beam splitter 4 is deflected by the first beam splitter 4 so as to travel along a path different from the path through which the incoming laser beam has passed through the first beam splitter 4. The laser beam deflected by the first beam splitter 4 is indicated by L1 and travels towards the second beam splitter 21. The reflected laser beam L1 entering the second beam splitter 21 is in part deflected so as to travel towards the photo-detecor assembly 28 and in part passed therethrough towards the polarizing beam splitter 22.
The reflected laser beam traveling from the second beam splitter 21 towards the photo-detector assembly 28 as indicated by L6 is detected by the photo-detector assembly 28, an electric output from which assambly 28 is supplied to the error detector 31 for the detection of the focusing error and/or the tracking error. Should one or both of the focusing and tracking errors occur, the error detector 31 provides an output signal to a servo system (not shown) well known to those skilled in the art which is designed to eliminated to focusing error and/or the tracking error. On the other hand, the reflected laser beam having passed through the second beam splitter 21 and traveling towards the polarizing beam splitter 22 as indicated by L7 is in part passed therethrough towards the photo-detector 26 as indicated by L8 and in part deflected by the polarizing beam splitter 22 so as to travel towards the photo-detector 27 as indicated by L9. The intensity of each of the laser beams L8 and L9 incident upon the photo-detectors 26 and 27, respectively, varies with a change in polarizing state of the reflected laser beam L1 and, therefore, information read out from the magneto-optical medium 6 can be picked up by detecting the difference between the outputs from the photo-detectors 26 and 27 with the use of the differential detector 32 to which they are supplied.
However, in the prior art magneto-optical head assembly, since one and the same reflected laser beam L6 is used for the detection of the focusing error and that of the tracking error, a cross-talks tends to occur between these two detection systems, posing a problem in that one or both of the focusing and the tracking tends to become insecure.
In order to eliminate the above discussed problem, the magneto-optical head system wherein the reflected laser beam L6 is further divided into two components for use in the detection of the focusing error and that of the trcking error has been proposed, such as shown in FIG. 2 and FIG. 3.
According to the system shown in FIG. 2, a third beam splitter 23 is disposed on the path of travel of light from the second beam splitter 21 towards the photo-detector 29. This third beam splitter 23 is utilized to divide the laser beam L6 into two light components L10 and L11; one component traveling towards and detected by a third photo-detector 29 and the other component traveling towards and detected by a fourth photo-detector 40. An output from the photo-detector 29 is supplied to a focusing error detector 34 whereas an output from the photo-detector 30 is supplied to a trcking error detector 33. In this circuit construction, since the separate detectors 34 and 33 are used for the detection of the focusing error and the tracking error, respectively, no problem associated with the signal cross-talk occur.
In the system shown in FIG. 3, arrangement has been made that the laser beam reflected from the magneto-optical medium 6 is allowed to be in part deflected by the first beam splitter 4 so as to travel towards the polarizing beam splitter 22 and in part passed therethrough towards a fourth beam splitter 24 disposed on the path between the first beam splitter 4 and the beam shaping prism 3 so that the reflected laser beam entering the fourth beam splitter 24 can be deflected, as the laser beam L12, towards the third beam splitter 23. In this arrangement, while the laser beam L1 deflected by the first beam splitter 4 is utilized for the information processing in a manner similar to that described with reference to FIG. 1, the laser beam L12 deflected by the fourth beam splitter 24 is divided into the beam components L10 and L11 by the third beam splitter 23 in a manner similar to that described with reference to FIG. 2 for the detection by the photo-detectors 29 and 30, respectively. Even in the system of FIG. 3, the separate detectors 34 and 33 are used for the detection of the focusing error and the tracking error, respectively, no problem associated with the signal cross-talk occur.
A further approach to eliminate the problem associated with the cross-talk is illustrated in FIG. 4. According to the system shown in FIG. 4, arrangement has been made that the reflected laser beam L1 entering the polarizing beam splitter 22 is divided by such polarizing beam splitter 22 into the light components L8 and L9 which are respectively detected by the photo-detectors 30 and 29 which are connected with the tracking error detector 33 and the focusing error detector 34. Respective outputs from the tracking and focusing error detectors 33 and 34 are supplied to a comparator 32 operable to provide an output signal indicative of the difference in intensity between the light components L8 and L9 for reading information recorded on the magneto-optical medium 6.
In any one of the prior art magneto-optical head systems shown in and described with reference to FIGS. 1 to 4, respectively, since the detection of the focusing condition and the tracking condition requires the use of a plurality of beam splitters 21 to 24 for dividing the reflected laser beam L1 into a corresponding number of light components, not only does the magneto-optical head assembly tend to become bulky, but also the use of the expensive optical elements makes the manufacture costly.
In particular, in the system shown in and described with reference to FIG. 1, since one and the same beam is used for the detection of the focusing error and that of the tracking error, the problem associated with the cross-talk occurs. Although any one of the systems shown in FIGS. 2 and 3, respectively, is effective to eliminate this problem, the extra beam splitter is required in the system of any one of FIGS. 2 and 3 as compared with the system shown in FIG. 1, making the system as a whole bulky and expensive.
On the other hand, in the system of FIG. 4, although the magneto-optical head assembly can be made compact as compared with that shown in and described with reference to any one of FIGS. 2 and 3, it has been experienced that, since the intensity of the laser beam L8 which has been passed through the polarizing beam splitter 22 and the intensity of the laser beam L9 which has been deflected by the polarizing beam splitter 22 tend to vary in correspondence with the change in level of the signal recorded on the magneto-optical medium 6, the intensity of the beam used for the detection of the focusing error and that for the detection of the tracking error vary correspondingly to such an extent as to result in the unstable signal processing performed by the focusing and tracking error detectors 34 and 33.
Apart from the above, it is well known that the diffraction grating having a grating pitch not greater than the wavelength of light exhibits a diffraction efficiency dependent on the polarization of the incident light, such as discussed by Mohara, et al., Appl. Optics, Vol. 23, No. 18, p. 3214,15th Sep. 1984. A series of experiments conducted with the use of the diffraction grating wherein the grating pitch .LAMBDA. is 0.5 micrometers have shown that, as shown in FIG. 5, when the wavelength .lambda. of the light incident upon such diffraction grating is 780 nm, S polarized light and P polarized light have given respective diffraction efficiencies which are considerably different from each other.
In view of the foregoing, the substitution of the diffraction grating for the polarizing beam splitter 22 employed in the system of FIG. 1 for separating the reflected laser beam L1 deflected from the first beam splitter 4 would eliminate the use of the expensive optical element. However, the mere replacement of the polarizing beam splitter with the diffraction grating tends to increase the bulkiness of the magneto-optical head assembly as a whole with no number of necessary component parts minimized.
The above discussed problem may be found in other applications than the magneto-optical head assembly. In other words, considering a problem inherent the polarizing optical element itself operable to reflect a portion of the laser light and to transmit therethrough the remaining portion of the same laser light, only one reflected light and only one transmitted light are available therefrom and, therefore, the systems will require the use of an increased number of polarizing optical elements if a large amount of information is desired to be handled. On the other hand, considering a problem inherent in the polarization analyzing device in which the differential detector is provided for detecting the difference in intensity between light components separated by the polarizing optical element, the system will require the use of an increased number of polarizing optical elements if a large amount of information is desired to be handled.